How Natural Draft Cooling Towers Work (Stack Effect)

Learn how natural draft cooling towers work! This 3D animated video shows you exactly how a natural draft cooling tower works! You will learn all of a natural draft cooling tower's main parts, how it works and some of its design features.

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How Natural Draft Cooling Towers Work
Natural draft cooling towers (‘natural convection cooling towers’) use the principle of convective flow to provide air circulation.

Natural draft towers are usually very tall in order to induce adequate air flow, they are also expensive to construct and are only used for applications where a large constant cooling requirement over many years is required; a thermal power plant is one such application. We will use a thermal power plant natural draft cooling tower to describe how this type of tower works.

Cool cooling water is pumped from the cooling tower basin to the power plant. The cool cooling water is heated by the process and its temperature increases. The warm cooling water is now pumped back to the cooling tower to be cooled.

The incoming warm water is distributed through spray nozzles inside the tower. The spray nozzles spray the warm water evenly over the entire fill. Water passes downwards through the fill whilst air passes upwards. As the water travels through the fill, some of it evaporates which causes the remaining water to be cooled; this is evaporative cooling. As air travels through the fill, its temperature increases, and it rises to the top of the cooling tower due to the stack effect (hot air is less dense than cool air and thus rises above it). The air exiting the top of the tower draws in more air at the base of the tower, creating a natural air flow from the base to the top of the tower; this is the stack effect and it is continuous providing cooling water is constantly circulated.

The purpose of the spray nozzles and fill is to increase the contact surface area between the water and air. A large contact surface area yields a much greater heat transfer capacity. All heat exchangers rely upon a large contact surface area between what is being cooled and the cooling medium.

Another important part of the cooling tower is the drift eliminator. The white plume exiting this cooling tower is not smoke, it is actually moisture, specifically it is the cooling water that has evaporated. Water losses associated with evaporation can be quite large; these water losses are referred to as drift. In order to reduce these losses, a drift eliminator is used.

Drift eliminators consist of parallel blades arranged on the air discharge side of the tower to remove entrained water droplets from the exiting air stream. The shape of the drift eliminator requires the entrained water droplets to change direction several times prior to being discharged out of the tower. Air has little problem changing direction and passing through the drift eliminator, but water droplets impinge upon the drift eliminator, condense, then drip back down onto the fill, then return to the cooling tower basin.

Why do cooling towers have such weird shapes?
You might be wondering why natural draft cooling towers have such a unique shape. Well, there are two reasons. The first reason is that the shape reduces the amount of construction material required when building such a large tower. The second reason that the paraboloid shape of the tower accelerates the air flow through the tower, which increases the tower’s cooling capacity.

What is the stack effect?
When air is heated, it becomes less dense. Because warm air is less dense than cold air, it will rise above cold air due to the density difference. This process of warm and cold air separating based upon densities is known as the stack effect or chimney effect.

For example, a hot air balloon flies in the air due to the hot air that is contained within the balloon. The hot air is less dense than the surrounding ambient air, which is why the hot air rises, pulling the balloon up with it.

This videos hows how natural draft cooling towers use evaporative cooling and the stack effect to reject heat.
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Видео How Natural Draft Cooling Towers Work (Stack Effect) канала saVRee